For ten days this summer, the great geographical and cultural distance that lies between the heart of Europe and the far reaches of Asia is being reduced to the length of a leisurely afternoon stroll on the National Mall. For the first time in its 36-year history, the Smithsonian Folklife Festival has a single &emdash; and remarkably ambitious &emdash; theme: the Silk Road. The name denotes the network of trade routes, over both land and sea, along which merchants and travelers began to move across Asia and Europe from the first millennium B.C.E. The most famous east-west component of the Silk Road began in Xi'an, the ancient capital of China, broke north ...